A steam turbine is a rotating device that converts thermal energy of steam into mechanical energy for driving an electrical generator, a pump, or any other rotary mechanical receiver. A “mechanical receiver” can be a mechanically driven device that absorbs power and performs work.
The turbine can include three modules: a high-pressure module, a medium-pressure module, and at least one low-pressure module. Steam supplied by a steam generator is first routed to the high-pressure module and then to the medium-pressure and low-pressure modules.
The high-pressure module includes a plurality of stages each provided with a set of fixed blades and a set of moving blades installed on a rotor. Each stage has two functions, expanding the steam, which corresponds to converting the thermal energy of the steam into kinetic energy, and converting the kinetic energy into mechanical energy by the set of moving blades.
A turbine can be sized for a maximum thermal power of the steam generator. It may be required to increase this maximum thermal power, for example, if it is desired to increase the electrical power supplied by the electrical generator driven by the turbine. In this case there is a change from a lower maximum thermal power to a higher maximum thermal power. The turbine should, then be adapted to this increased maximum thermal power.
To do this it is known to modify the active parts of the turbine to accept a greater steam flow rate. The active parts of the turbine are the parts allowing the steam to expand, for example, the sets of fixed blades and the sets of moving blades attached to the rotor.
This process can be long and costly, as it can involve changing the rotor with its moving blades and the sets of fixed blades.
A second method includes anticipating an increase in maximum thermal power and designing the turbine accordingly, for example, by designing the turbine for the higher maximum thermal power and providing a device for limiting the steam flow rate to operate at a lower maximum thermal power. In a first variant of this second method, it is possible to operate at the lower maximum thermal power by limiting the overall steam flow rate by steam inlet valves. In a second variant of this method, it is possible to operate at the lower maximum thermal power by reducing the steam flow rate through one sector of the first set of fixed blades of the high-pressure module.
This second method can reduce the efficiency of the turbine.